Alcohols may be obtained by catalytic hydrogenation of aldehydes which have been obtained, for example, by hydroformylation of olefins, often referred to as the oxo or oxonation process or oxo synthesis. Large quantities of alcohols are used as solvents and as intermediates for preparing many organic compounds. Important downstream products of alcohols are plasticizers and detergents.
Aldehydes may be catalytically reduced with hydrogen to form alcohols. Catalysts which include at least one metal of groups 1b, 2b, 6b, 7b and/or 8 of the Periodic Table of the Elements are frequently used. The hydrogenation of aldehydes may be carried out continuously or batchwise using catalysts in the gas or liquid phase.
For example, U.S. Pat. No. 6,680,414 (equivalent to EP 1 219 584 B) discloses a process comprising in a homogeneous liquid phase comprising water, and over a fixed-bed catalyst, continuously hydrogenating at least one hydroformylation product obtained directly from a hydroformylation of one or more C4-16 olefins to produce at least one output mixture: wherein said fixed-bed catalyst comprises at least one element of transition group eight of the Periodic Table of the Elements; wherein said output mixture comprises at least one corresponding alcohol and from 0.05 to 10% by weight of water; and wherein in a steady-state operation of the process, from 3 to 50% more hydrogen is fed to the hydrogenation than is consumed by the hydrogenation.
Other background references include GB 2 142 010, DE 198 42 370, DE 2 628 987, DE 198 42 370, DE 102 41 266, WO 2001/97809, WO 2005/058782, EP 3 192 08 A, U.S. Pat. Nos. 2,809,220, 4,401,834, 5,059,710, and 5,306,848.
For the industrial production of alcohols by hydrogenation of aldehydes from the oxo process, preference is given, especially in the case of large-volume products, to continuous gas or liquid phase processes using catalysts located in a fixed bed.
Compared to gas-phase hydrogenation, liquid-phase hydrogenation has a more favorable energy balance and gives a higher space-time yield. As the molar mass of the aldehyde to be hydrogenated increases, i.e., as the boiling point increases, the advantage of the more favorable energy balance increases. Higher aldehydes having more than 6 carbon atoms, preferably, from 6 to 15 carbon atoms, are hydrogenated in the liquid phase.
However, hydrogenation in the liquid phase has the disadvantage that, owing to the high concentrations of both aldehydes and alcohols, the formation of what U.S. Pat. No. 6,680,414, terms “high boilers” via subsequent and secondary reactions is promoted. Thus, aldehydes can more readily undergo aldol reactions (addition and/or condensation) and form hemiacetals or acetals with alcohols. The acetals or hemiacetals formed can undergo elimination of alcohol or water, respectively, to form (unsaturated) ethers which are hydrogenated under the reaction conditions to form saturated ethers. These secondary by products thus reduce the yield. Industrial aldehyde mixtures which are used for the hydrogenation frequently already contain varying concentrations of “high boilers.”
For example, hydroformylation of olefins in the presence of cobalt catalysts gives crude aldehydes which contain esters of formic acid (formates) and also aldol products, high esters, and ethers as well as acetals as “high boilers.”
For some commercial processes utilizing a hydroformylated product as a raw starting material, the hydrogenation feed, i.e., the feed to be hydrogenated, may contain up to 15 wt % of formates. When in contact with the hydrogenation catalyst, for example, copper chromite, sulphided nickel, moly catalyst, or nickel catalyst, the formates decompose to form the oxo alcohol, CH3OH (methanol), H2 (hydrogen), and CO2 (carbon dioxide). Methanol formation is favored when the water content of the hydrogenation feed is low and the reactor temperature and pressure are high. Alternatively, CO2 and H2 formation is favored when the water content of the reactor is close to 3 wt % and the temperature and pressure of the reactor are on the low side, for example, from 100-200° C. at 10-60 bar. The hydrogen generated from the decomposition may be useful for the hydrogenation reaction of aldehydes under certain conditions. However, the production of too much methanol is undesirable because it may end up in plant waste water. The following reaction schedules are also provided for further illustration.R—CH2—OOCH+H2O→R—CH2OH+HCOOH  (I)HCOOH→CO2+H2  (II)R—CH2—OOCH+2H2→R—CH2OH+CH3OH  (III)
Additionally, without being bound to theory, the methanol may promote the formation of by products such as methyl nonanoate by an esterification reaction with nonanoic acid which is present in the hydrogenation feed.
Thus, there exists the need to provide a hydrogenation process that utilizes hydrogen produced from the decomposition of formates while minimizing the amount methanol and its resulting by products also produced from the same decomposition or from subsequent reactions.